Actually the alkenes are the compounds that have a double bond between two carbon atoms. In the formulas given above in case of ethene we have two carbon atoms but in case of methene (just supposing actually it can’t even exist) there isn’t any second carbon atom with which it could make a double bond.
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The P2 molecule of phosphorus is not found because of the larger size of the phosphorus atom. The phosphorus is so larger that the p-orbitals which are required to overlapp sideways , are unable to overlapp sufficiently to make a pie bond. Below is the structure of oxygen atom that is showing π-bond But phosphorus
The Unconditional Guru 